I wonder why, while Firefox has now integrated search for addons and
better addons site is planned...

I don't think we should package extensions, but what do you think about
that situation in U?

This may make most sense for binary extensions where we may want to
compile it so users don't need to install compat-libstdc++33 and
optimize using the latest greatest compiler, and our opt flags du jour.

However, one thing which is highly relevant is that extensions usually
have no discernable upstream (unless you count a.m.o), sometimes do not
include license information or have hostile licenses, and often don't
include source code. *.xpi files are not source code as they can
contain binary data, such as compiled byte code (*.xpt) or .so files